Field of the Invention
The invention initially relates to a method for determining the flow of a multi-phase medium flowing through a measuring tube with a nuclear magnetic flowmeter having a pre-magnetization device and a measuring device, wherein the pre-magnetization device implements at least two pre-magnetization sections of differing lengths with the lengths L1, L2, . . . . Furthermore, the invention relates to a nuclear magnetic flowmeter, in particular for use in conjunction with the method according to the invention.
Description of Related Art
Nuclear magnetic flowmeters, which are designed for determining the flow of a medium flowing through a measuring tube, are used, in particular, when the medium contains several phases. Determining the flow of a multi-phase medium also includes, in addition to determining flow velocity and flow rate, determining the portions of the individual phases of the medium. However, determining the flow of a single-phase medium is also possible using a nuclear magnetic flowmeter.
The prerequisite for applicability of nuclear magnetic measuring methods is that the medium or each phase of the medium has atomic nuclei with magnetic moments.
In order to differentiate between individual phases, it is additionally necessary that the individual phases have differentiable relaxation times. Presently, the so-called spin-lattice relaxation time T1, also called T1 time, is of particular importance.
If a system containing an atomic nucleus bearing a magnetic moment is located in an external magnetic field exhibiting a specific direction, then the magnetic moments of the atomic nuclei are aligned in the external magnetic field. The magnetic moments take up a state that is parallel or anti-parallel to the external magnetic field, wherein there is a higher probability that the state parallel to the external magnetic field is filled, so that a “net magnetization” parallel to the external field is formed in the system. This “net magnetization” is also called equilibrium magnetization. The magnetization can be deflected out of its equilibrium position by an external interference. However, as soon as the interference is gone, the magnetization strives to return to its equilibrium position, to relax again in its equilibrium position. The time that is necessary for this to take place is the spin-lattice relaxation time T1.
The spin-lattice relaxation time T1 has a different value for each phase in a multi-phase medium, as already described above. Accordingly, the T1 time is an important distinguishing and characterizing parameter of each phase. In particular, the knowledge of the T1 time is a good basis for making complete characterization of the medium possible. Determining the T1 time is often relatively complicated and, above all, very time-consuming.
It is to be pointed out here that multi-phase mediums extracted from oil sources consist essentially of the two liquid phases crude oil and saltwater, and of the gaseous phase natural gas, wherein all three phases contain hydrogen nuclei that bear a magnetic moment.
Nuclear magnetic flowmeters known from the prior art, which are used for determining the flow of a multi-phase medium flowing through a measuring tube, can be designed so that they include a pre-magnetization device and a measuring device, wherein the pre-magnetization device implements at least two pre-magnetization sections with different lengths L1, L2, . . . .